I. Field of the Invention
This invention relates to a method of producing platelets of refractory metal borides and to the platelets so-produced.
II. Description of the Prior Art
Ceramic materials in the form of discrete particles are of increasing interest nowadays because of their ability to reinforce composite materials having metal or ceramic matrices. Refractory hard borides are of particular interest in this regard because of their high strength, inertness and ability to resist high temperatures. When such materials are in the form of platelets (single crystals which grow preferentially in two dimensions and have aspect ratios of at least 5) in the size range of 10-20 microns, a very strong reinforcing effect can be expected because of the non-spherical nature of the particles.
TiB.sub.2 is an especially useful reinforcing material for metal matrices containing aluminum because TiB.sub.2 is one of the few refractory materials which are both wettable by molten aluminum and unreactive with it. However, platelets of this material are not currently commercially available. Even when platelets of suitable refractory metal borides are available, they tend to be very expensive, and are often contaminated with other types of particles, such as spherical particles or whiskers which may not be desirable for matrix reinforcement.
A variety of methods are known for the preparation of refractory metal borides in the form of particles. For example, it is known that titanium diboride can be produced by a carbothermal process by reacting titanium dioxide, boron carbide and carbon at high temperature. The reaction involved is illustrated below: EQU 2TiO.sub.2 +B.sub.4 C+3C.fwdarw.2TiB.sub.2 +4CO
The equilibrium temperature of this process is 886.degree. C. but, in practice, the temperature has to be maintained well above this level in order to achieve sufficiently high reaction rates. Disadvantageously, reaction temperatures above 1800.degree. C. are generally required and platelets are rarely formed. The requirement for the prior synthesis of the boron carbide also reduces the attractiveness of the process.
TiB.sub.2 particles can also be produced by direct reaction between TiO.sub.2, B.sub.2 O.sub.3 and C. The reaction involved is shown below: EQU TiO.sub.2 +B.sub.2 O.sub.3 +5C.fwdarw.TiB.sub.2 +5CO
The equilibrium temperature for this reaction is 1365.degree. C. but, in practice, much higher temperatures (again above 1800.degree. C.) are required in order to achieve satisfactory conversion in a reasonable time. This may be because intermediate titanium oxycarbides are formed, which require temperatures above 1800.degree. C., and preferably close to 2000.degree. C., for the formation of TiB.sub.2. At these temperatures, B.sub.2 O.sub.3 volatilizes and it therefore becomes difficult to maintain the proper ratio of the starting materials, so the resulting product contains an excess of oxide or carbon. In view of this, this method of synthesis is not used in commercial production processes.
Other processes for the synthesis of TiB.sub.2 include gas phase synthesis by reactions between TiCl.sub.4, BCl.sub.3 and H.sub.2, but this is expensive and difficult to operate on a commercial scale.
In general, the products of the above reactions and similar reactions for the production of borides of other refractory metals tend to form products which comprise an agglomeration of particles having sizes and shapes which are not very suitable for matrix reinforcement.
Prior patents and articles which describe the formation of titanium diboride and borides or carbides of other refractory metals include the following: (1) Kim, J. J and McMurtry, C. H., Ceramic Engineering Sci. Proc., 1985, 6 (9-10): (2) Walker, J. K., Advanced Ceramic Materials 1988, 3 (6), 601-4; (3) Japanese patent publication JP 43-2216 to Doi, Y.; (4) Mokoto, K. et al, Osaka Kogyo Gijutsu Shikenjo Kiho, 18(1), 72-7; (5) U.S. Pat. No. 4,812,425 to Walker Jr.; (6) U.S. Pat. No. 3,328,127 to Byrns; (7) U.S. Pat. No. 3,351,429 to Timms; (8) U.S. Pat. No. 3,249,401 to Wood et al; (9) U.S. Pat. No. 4,376,029 to Joo et al; (10) U.S. Pat. No. 4,439,382 to Joo et al; (11) U.S. Pat. No. 4,595,545 to Sane; and (12) U.S. Pat. No. 3,379,647 to Smudski. These patents and articles do not disclose processes which can be used reliably, and on a commercial scale, for the formation of refractory platelets.
There is therefore a need for improved methods of producing borides of titanium and other refractory metals which can result in the formation of platelets.
An object of the present invention is consequently to provide a method of producing such products.